To a solution of D8 (0.301 g, 0.992 mmol) in acetic acid (4.5 ml) were added 4-phenylpiperidine (0.160 g, 0.992 mmol) and formaldehyde (0.223 ml, 0.992 mmol; 37%). The resulting mixture was heated in a sealed tube at 80° C. for 16 h. The reaction mixture was diluted with DCM and washed with 2M NaOH. The organic layer was separated, dried (MgSO4) and concentrated in vacuo. The crude product thus obtained was purified by column chromatography (silica gel; DCM/7M solution of NH3 in MeOH up to 2% as eluent). The desired fractions were collected and concentrated in vacuo to yield a residue that was purified by HPLC (C18Xbridge 19×100; mobile phase AcONH4/CH3CN gradient as eluent). The desired fractions were collected and concentrated in vacuo to yield final compound E12 (0.085 g, 18%).
